The invention relates to a polycrystalline silicon rod and to a process for production of a polycrystalline silicon rod.
Polycrystalline silicon (polysilicon for short) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone-melting (FZ) processes, and for production of mono- or polycrystalline silicon by various drawing and casting processes for production of solar cells for photovoltaics.
Polycrystalline silicon is generally produced by means of the Siemens process.
In this process, in a bell-shaped reactor (“Siemens reactor”), support bodies, typically thin filament rods (thin rods) of silicon, are heated by a direct passage of current, and a reaction gas comprising hydrogen and one or more silicon-containing components is introduced.
Typically, the silicon-containing component used is trichlorosilane (SiHCl3, TCS) or a mixture of trichlorosilane with dichlorosilane (SiH2Cl2, DCS) and/or with tetrachlorosilane (SiCl4, STC). The use of monosilane (SiH4) is also known.
The thin rods are typically inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply. Every two thin rods are coupled via a horizontal bridge (likewise made from silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the support bodies.
High-purity polysilicon is deposited on the heated thin rods and the horizontal bridge, as a result of which the diameter thereof grows with time.
The deposition process is typically controlled by the setting of rod temperature and reaction gas flow rate and composition.
The rod temperature is measured with radiation pyrometers, usually on the surfaces of the rods facing the reactor wall.
The rod temperature is set by control or regulation of the electrical power, either in a fixed manner or as a function of the rod diameter.
The amount and composition of the reaction gas are set as a function of the time or rod diameter.
After a desired diameter has been attained, the deposition is ended and the polysilicon rods which have formed are cooled to room temperature.
The morphology of the growing rod is determined by the parameters of the deposition process.
The morphology of the deposited rods may vary from compact and smooth up to very porous and fissured material.
U.S. Pat. No. 6,350,313 B2 discloses the further processing of compact polycrystalline silicon rods.
Compact polycrystalline silicon is very substantially free of cracks, pores, gaps, fissures etc.
The apparent density of such a material corresponds to the true density of polycrystalline silicon and is 2.329 g/cm3.
US 2003/0150378 A2 discloses “teardrop poly” and a process for production thereof. In this process, a compact, hole-free, high-purity polysilicon rod is deposited from monosilane SiH4 by means of the Siemens process up to a silicon rod diameter of 45 mm at 850° C. and a silane concentration of 1.14 mol %. Subsequently, the rod surface temperature is increased instantly from 850 to 988° C. and the silane concentration reduced instantly from 1.14 to 0.15 mol %. This parameter jump instantly alters the growth of the silicon crystals on the silicon rod, and needles, called dendrites, grow out of the rod surface. These dendrites can subsequently be removed from the compact rod part, while the compact part has to be processed further separately.
US 2010/219380 A1, in contrast, discloses a polycrystalline silicon rod having an apparent density in the range from 2.0 to 2.3 g/cm3 and an overall porosity of 0.01 to 0.2. The silicon rod has a similar structure, though this structure contains pores, gaps, crevices, cracks and fissures. Such a polycrystalline silicon rod can be comminuted into chunks with comparatively low energy expenditure, and accordingly leads to less surface contamination at the surface of the chunks.
US 2010/219380 A1 likewise discloses a process for producing a polysilicon rod as claimed in any of claims 1 to 3, in which a stream of a reaction gas comprising a chlorosilane mixture and hydrogen is introduced into a reactor and high-purity polysilicon is deposited on a filament rod of silicon heated by direct passage of current, the filament rod being formed from two vertical rods and one horizontal rod, and the horizontal rod forming a linking bridge between the vertical rods, characterized in that the chlorosilane mixture used is a mixture of di- and trichlorosilane and the passage of current through the filament rod is regulated such that the filament rod has a temperature at the underside of the bridge between 1300 and 1413° C. and the temperature of the reaction gases in the reactor is measured and adjusted so as not to be more than 650° C., and the flow rate of the chlorosilane mixture is adjusted to its maximum value within less than 30 hours, preferably within less than 5 hours, from the start of supply of the chlorosilane mixture.
The compact rods are more expensive to produce. The deposition process is slower. However, compact rods generally lead to better yields in subsequent crystallization steps.
The increase in the base parameters of rod temperature, specific flow rate, silane concentration generally leads to an increase in the deposition rate and hence to an improvement in the economic viability of the deposition process.
However, natural limits are set on each of these parameters, the exceedance of which disrupts the production process.
If, for example, the concentration of the silicon-containing component selected is too high, there may be homogeneous gas phase deposition.
The effect of an excessively high rod temperature may be that the morphology of the silicon rods to be deposited is not compact enough to provide a sufficient cross-sectional area to the current flow, which rises with the growing rod diameter. If the current density becomes too high, this can cause the melting of silicon. From a certain diameter of about 120 mm, even in the case of compact morphology, silicon in the rod interior can become liquid, since high temperature differences exist between surface and rod center.
This is also problematic in the process according to US 2003/0150378 A2, since the current flows exclusively through the compact part of the silicon rod. If the diameter of the compact part selected is too low, which is actually desirable since the aim of the process is the production of dendrites, there is a risk of melting. With rising diameter, higher currents are required, and so the diameter of the compact part must also increase. This reduces the yield of dendrites.
In the case of a polycrystalline silicon rod according to US 2010/219380 A1, in contrast, a majority of the rod cross section is available for current flow. The electrical conductivity is not impaired by the small cracks and pores compared to conventional compact silicon.
For most applications, polycrystalline silicon rods have to be crushed into smaller chunks. Typically, the chunks are subsequently classified by size. A process for comminuting and sorting polysilicon is described, for example, in U.S. Pat. No. 8,074,905 B2. In general, it is immaterial here whether the polycrystalline silicon is in compact or brittle form.
The morphology of polycrystalline rods and of chunks obtained therefrom, however, has a strong influence on the performance of the product.
As mentioned above, compact rods show better yields in crystal pulling.
A porous and fissured morphology like that according to US 2010/219380 A1, in contrast, has adverse effects on the crystallization characteristics. This particularly affects the demanding CZ process, in which it has not been possible to date to use porous and fissured chunks owing to the economically unacceptable yields.
U.S. Pat. No. 7,939,173 B2 discloses a polysilicon rod which, in the radial cross section, has regions with different crystal structures, an inner structure comprising few acicular crystals, if any, and an outer structure comprising acicular crystals and microcrystals, with presence of a mixed zone in which there is a fluid transition from the inner structure to the outer structure. This polysilicon rod is intended for use in the FZ process. The production is effected by deposition of silicon from hydrogen-diluted chlorosilanes having a molar proportion of the chlorosilanes of not more than 30% on a filament rod of silicon at a rod temperature of 950 to 1090° C. at the start of deposition. To obtain the different crystal structures, the process parameters are altered in a fluid manner. The rod temperature is lowered and the amount of hydrogen injected reduced, such that the molar proportion of the chlorosilanes is increased to 35-60%.
The problem described gave rise to the objective of the present invention.
The aim was to provide polycrystalline silicon which is less expensive to produce than compact material but nevertheless exhibits good performance in CZ crystal pulling.